The Tether cryptocoin ("USDT") is backed by US dollars. There is 1 USD deposited for each USDT.
You could do just the same with physical gold.
This has worked wonderfully in the past, until Wells-Fargo decided to block all transactions to and from Tether, allegedly for money-laundering reasons.
Now 1 USDT is traded for around 0.90 USD as holders are starting to get anxious because the situation hasn't been resolved for weeks.
This, in my opinion, has also indirectly increased demand for Bitcoin, driving its price up.